1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly, to an in plane switching mode liquid crystal display device having ferroelectric liquid crystal material and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display (LCD) device controls an electric field applied to a liquid crystal cell. The controlling of the electric field modulates light incident to the liquid crystal cell, thereby displaying a picture. A liquid crystal material injected into the liquid crystal display device is in a middle state of a solid and a liquid having both fluidity and elasticity together.
A twisted nematic (TN) mode uses a vertical electric field scheme and is the liquid crystal mode most commonly used in liquid crystal display devices until recently. The TN mode has the advantage of having a relatively high aperture ratio. On the other hand, the TN mode has a disadvantage of not having a wide viewing angle because of the refractive index of the liquid crystal material. Further, the response speed of the liquid crystal material using TN mode is slow.
An in-plane switching (IPS) mode uses an electric field that is parallel to the display panel of the liquid crystal display device. In the IPS mode, an electric field is formed between electrodes formed on a substrate, and liquid crystal molecules are driven by the horizontal electric field. FIG. 1 is a cross-sectional view schematically illustrating schematically an in-plane switching mode liquid crystal display device according to the related art. In the IPS mode, as shown in FIG. 1, a pixel electrode 16 and a common electrode 15 are formed on a lower glass substrate 18, and an electric field 20 is formed in a horizontal direction by a voltage difference applied between the electrodes 15 and 16. Liquid crystal molecules 14 are rotated on a surface of the substrate by the electric field 20 to modulate a polarization component of light transmitting a liquid crystal layer. In FIG. 1, polarizers 11 and 19 are respectively attached to an upper glass substrate 12 and the lower glass substrate 18 such that the axes of the polarizers cross each other. The alignment films 13 and 17 are respectively formed on the upper glass substrate 12 and the lower glass substrate 18. If the polarization component of the light transmitting through the liquid crystal layer is changed by 90 degrees, then the light passes through the upper polarizer 11. On the other hand, if the polarization component of the light does not change, then the light cannot pass thorough the upper polarizer 11.
The IPS mode liquid crystal display device shown in FIG. 1 has an advantage in that it has a wide viewing angle since a refractive index change of the liquid crystal material is not large. However, in the IPS mode liquid crystal display device, the electric field applied to the liquid crystal molecules is done with opaque electrodes 15 and 16 on the lower substrate. As a result, it has the disadvantage of having a low aperture ratio.
A ferroelectric liquid crystal (FLC) has the advantage having a high response speed and a wide viewing angle. The ferroelectric liquid crystal has a structure the uses both electrical and magnetic properties. A ferroelectric liquid crystal can be driven within plane that rotates along a virtual cone in response to electric field. The ferroelectric liquid crystal has a permanent polarization, in other words, a spontaneous polarization without having an external electric field. Like an interaction between two magnets. If an external electric field is applied, the ferroelectric liquid crystal rotates rapidly with an interaction between the external field and the spontaneous polarization. The response speed of the ferroelectric liquid crystal is several hundred or thousand times as fast as that of other mode liquid crystal. Further, since the ferroelectric liquid crystal has an in-plane-switching property in itself, it has a wide viewing angle without having a special electrode structure or a compensation film. Ferroelectric liquid crystal is classified into a V-switching mode and a half V-switching mode according to a characteristic reacting in response to a polarity of an electric field.
In the ferroelectric liquid crystal cell of the V-switching mode, as temperature is lowered, a thermodynamic phase transition arises like an isotropic→a smectic A phase (SA)→a chiral smectic X phase (Sm X*)→a crystal. Isotropic is a state where the liquid crystal molecules do not have direction and location order. Smectic A phase is a state where the liquid crystal molecules are divided into a virtual layer and arranged vertically on the virtual layer, and has a symmetry about up and down. The chiral smectic X phase is a middle state between the smectic A phase and the crystal phase. FIG. 2 is a graph illustrating a voltage vs. a transmittance property of a ferroelectric liquid crystal of a V-switching mode according to the related art. The ferroelectric liquid crystal cell of the V-switching mode in which the liquid crystal molecule is phase-transited to the chiral smectic X phase, as shown in FIG. 2, improves a light transmittance of an incidence light by changing the arrangement state of the liquid crystal molecules in response to the external voltage of positive polarity +V and the external voltage of negative polarity −V.
The V-switching mode has the advantages of high-speed-response and wide viewing angle. However, the V-switching mode has the disadvantages of requiring high power for driving a liquid crystal cell to overcome a large spontaneous polarization value and a large storage capacitor to store enough charge for maintaining a data voltage. Accordingly, if the V-switching mode is used in a liquid crystal display device, power consumption of the liquid crystal display device is high, and an aperture ratio is reduced because of a large electrode area for a capacitor.
In contrast, the half V-switching mode has the advantages of high-speed-response, wide viewing angle, and further, it is well suited for displaying moving picture and representing liquid crystal display device because the capacitance is low comparatively. FIG. 3 is a diagram illustrating a phase transition process of the ferroelectric liquid crystal of a half V-switching mode according to the related art. As shown in FIG. 3, the phase transition from the isotropic to the chiral nematic phase (N*) occurs below the transition temperature (Tni), the phase transition from the chiral nematic phase (N*) to the chiral smectic C phase (Sm C*) occurs below the transition temperature (Tsn), and the phase transition from the chiral smectic C phase to the crystal occurs as the temperature is lowered below the transition temperature (Tcs) causing. The thermodynamic phase transitions that can be attained are isotropic→the chiral nematic (N*)→the chiral smectic C phase (Sm C*)→the crystal.
FIG. 4 is a diagram illustrating a change of molecule arrangement depending on whether or not an alignment under an electric field is applied to the ferroelectric liquid crystal of the half V-switching mode according to the related art. A method of manufacturing the liquid crystal cell of a half V-switching mode will be described as follows with reference to FIG. 4. The ferroelectric liquid crystal is injected into the cells arranged in parallel at an incipient temperature of the isotropic phase that does not have direction and location order. If the temperature of the isotropic is lowered to a designated temperature, the ferroelectric liquid crystal becomes chiral nematic phase (N*) arranged in parallel with respect to the rubbing direction. In the chiral nematic phase (N*), if the temperature is gradually reduced and a sufficient electric field is applied to the inside of the liquid crystal cell, then the ferroelectric liquid crystal of the chiral nematic phase (N*) transits to the chiral smectic phase (SmC*) and the spontaneous polarization direction of the ferroelectric liquid crystal is arranged coincidentally with a direction of an electric field formed inside the cell. As a result, the spontaneous polarization direction of the ferroelectric liquid crystal coincides with the direction of electric field applied upon electric field alignment among two arrangement directions of the molecules depending on the alignment process of an upper plate and a lower plate when the parallel alignment is disposed, and the ferroelectric liquid crystal has a uniform alignment condition entirely by virtue of the electric field alignment, as shown in FIG. 4.
FIGS. 5A and 5B are graphs illustrating the changes of light transmittance according to the voltage in the ferroelectric liquid crystal cell of the half V-switching mode, respectively. Referring to FIG. 5A, in case of the liquid crystals being aligned under electric field by a negative polarity voltage −V, the ferroelectric liquid crystal cell of the half V-switching mode makes incident light transmit by changing the polarization direction of the incident light 90° only when a positive voltage +V is applied thereto, and makes most of the incident light cut-off by maintaining the polarization direction of the incident light when the negative voltage −V is applied thereto. The ratio of the light transmittance increases in proportion to the positive electric field intensity and maintains the maximum value if the positive electric field intensity increases to more than the designated threshold value. In contrast, if the ferroelectric liquid crystal of the half V-switching mode cell is aligned under electric field by the positive polarity voltage +V, the ferroelectric liquid crystal cell of the half V-switching mode, as illustrated in FIG. 5B, makes the incident light transmit only when the negative voltage −V is applied thereto and makes most of the incident light cut-off when the positive voltage +V is applied.
FIG. 6 represents a change of the ferroelectric liquid crystal arrangement when applying the alignment electric field of a negative polarity E(−) to the ferroelectric liquid crystal cell of the half V-switching mode, and a change of the ferroelectric liquid crystal arrangement when respective external electric fields E(+) and E(−) of a positive polarity and a negative polarity are applied thereto. Referring to FIG. 6, if the ferroelectric liquid crystal cell of the half V-switching mode is aligned under the external electric field of the negative polarity E(−), then the spontaneous polarization direction Ps of the ferroelectric liquid crystal is aligned uniformly in a direction coinciding with the external electric field of the negative polarity (E−).
After the electric field is aligned as described above, if the external electric field of the positive polarity (E(+)) is applied to the ferroelectric liquid crystal cell of the half V-switching mode, an arrangement of the liquid crystal molecules is changed and a direction of spontaneous polarization Ps coincides with the external electric field of the positive polarity E(+). The polarization direction of the light incident to the liquid crystal layer via a lower plate polarizer is changed to the polarization direction of an upper plate polarizer by the liquid crystal molecules in which the arrangement is changed and the incident light transmits through the polarizer installed in the upper plate. If the external electric field of the negative polarity E(−) is applied or the external electric field is not applied to the ferroelectric liquid crystal cell of the half V-switching mode, then the arrangement of the liquid crystal molecules maintains an incipient arrangement state as it is and the incident light maintains the polarization direction. Thus, the incident light cannot pass through the polarizer in the upper plate.
FIG. 7 is a configuration illustrating two sub-regions existing in one liquid crystal cell in a case that an alignment under an electric field is not performed according to the related art. Without performing the electric field alignment process, the two states of molecule arrangements of which the layers are randomly vent appear while phase-transiting from the chiral nematic phase (N*) to the chiral smectic C phase (SmC*). If a random bi-stable state in which the molecule arrangement of the ferroelectric liquid crystal is random becomes, then it is difficult to uniformly control the ferroelectric liquid crystal. If the two states of molecule arrangements of which the layers are different from each other randomly exist in one ferroelectric liquid crystal cell, then the liquid crystal cell, in response to the electric field having polarities different from each other, becomes divided into two regions that are separately driven. More specifically, as shown in FIG. 7, if two molecule arrangements exist with the layers are randomly bent in the same ferroelectric liquid crystal cell, then the direction of the spontaneous polarization Ps of ferroelectric liquid crystal molecule becomes different in the two regions.
FIG. 8 is a configuration illustrating a liquid crystal molecule reacting by an exterior electric field in two sub-regions shown in FIG. 7 according to the related art. In FIG. 7, it is assumed that a symbol ⊙ represents a spontaneous polarization direction of the ferroelectric liquid crystal molecule identical to an electric field direction of a positive polarity and a symbol  represents a spontaneous polarization direction of the ferroelectric liquid crystal molecule identical to an electric field direction of a negative polarity. If the electric field of the negative polarity is applied to the ferroelectric liquid crystal cell including two molecule arrangements of which the layers, having the direction of the spontaneous polarization Ps different from each other, are different from each other, then the liquid crystal molecules having the direction of the spontaneous polarization Ps in running parallel to the electric field direction  of the negative polarity included in a molecular arrangement, as in the right region of FIG. 7, does not react to the electric field of the negative polarity and maintains the spontaneous polarization direction as it is. In contrast, the liquid crystal molecules included in a molecular arrangement, as in a left region in FIG. 7, reacts to the electric field of the negative polarity to rotate along the virtual cone, and, at the same time, the direction of the spontaneous polarization Ps is changed in the  direction in running parallel to the electric field of the negative polarity, as shown in FIG. 8. At this time, while an incident light passes through the left region of FIG. 7, the polarization direction is changed forward in a direction of a light outgoing side (that is, a polarization direction of the polarizer in the upper plate) to pass through the polarizer in the upper plate. In contrast, an incident light entered to the right region of in FIG. 7 maintains its polarization direction to enter the polarizer in the upper plate, whereby the incident light cannot transmit the polarizer in the upper plate.
Accordingly, the two regions exist with the layers are vent randomly and the direction of the spontaneous polarization Ps is different in the two regions in one ferroelectric liquid crystal cell, then it is impossible to accurately control the half V-switching mode. Further, as shown in FIG. 7, if there are exist the two regions different from each other in the same ferroelectric liquid crystal cell, then a brightness difference can be appeared every frame period because the liquid crystal molecules react to the electric fields having polarities different from each other depending on the respective regions.